Conventionally, there has been known a device for folding a sheet in a width direction to overlap one widthwise part of the sheet with another part while conveying the sheet in its longitudinal direction (e.g. Publication of Japanese Patent No. 2698791).
FIGS. 9A and 9B are perspective views enlargedly showing an essential part of a device described in the Publication of Japanese Patent No. 2698791, wherein FIG. 9A shows a state where a sheet is being folded and FIG. 9B shows the state of FIG. 9A with the sheet omitted.
The device of Publication Japanese Patent No. 2698791 includes an upstream side plate 30 and a downstream side plate 31 and a sheet S2 is conveyed in its longitudinal direction while being held in contact with the both plates 30, 31 as indicated by an arrow of FIG. 9A.
The upstream side plate 30 has an edge part 30a for forming a first folding line B5 of the sheet S2 and an edge part 30b for forming a second folding line B6 of the sheet S2.
The first folding line B5 is a folding line extending in the width direction from one widthwise end S21 to a widthwise intermediate part of the sheet S2.
The second folding line B6 extends obliquely to a conveying direction from the other widthwise end S22 of the sheet S2 at a position upstream of the first folding line B5 toward an end of the first folding line B5.
The downstream side plate 31 has an edge part 31a for forming a third folding line B7 for reversing one widthwise part of the sheet S2 such that the one widthwise part of the sheet S2 folded along the second folding line B6 overlaps with another part.
The third folding line B7 extends obliquely to the conveying direction from the end of the first folding line B5 toward the other widthwise end S22 of the sheet S2 to be reversed.
In the device described in the Publication of Japanese Patent No. 2698791, the plates 30, 31 are fixed to each other. That is, the respective edge parts 30a, 30b and 31a for forming the first to third folding lines B5 to B7 are fixed to each other.
Thus, there is a problem that an area of the sheet S2 on the side of the other widthwise end S22 located between the second and third folding lines B6, B7 is not suitable for folding and. Therefore, the sheet S2 cannot be folded precisely in the case of changing a thickness of the sheet S2 or changing a tension applied to the sheet S2.